The present invention relates generally to vehicle fuel tanks and, more particularly, to an assembly for eliminating unwanted fuel vapor emissions from vehicle fuel tanks.
Environmental concerns and governmental regulations require reduced atmospheric emissions of volatile hydrocarbon fuel vapor. One source of hydrocarbon fuel vapor is the fuel tanks of vehicles using gasoline or other hydrocarbon fuels with high volatility. The fuel vapor can escape to the atmosphere during the filling of the tanks and usually even after the tanks are filled.
Current production motor vehicles include a fuel storage tank to hold gasoline, a pump module to send liquid fuel to the engine and measure the quantity of fuel remaining and a carbon canister to catch and hold hydrocarbon vapor that would otherwise evaporate from the fuel tank out into the environment. Typically, the canister contains activated charcoal that captures fuel vapor formed in the fuel tank through a vapor tube and valve assembly mounted in the top of the tank. The canister also communicates with the intake manifold of the vehicle engine to exhaust fuel vapor stored in the canister to the manifold for combustion during operation of the engine. The valve assembly usually has a level responsive valve that enables the valve to stay open at a sufficiently low fuel level to permit fuel vapor to flow freely from the fuel tank into the canister. During refueling, as the fuel level rises to approach a desired maximum level of fuel in the tank, a float may be raised to close the level responsive valve to prevent liquid fuel from flowing into the vapor receiving canister.
The use of such onboard vapor recovery systems to remove excess fuel vapor from the fuel tank is one solution to the environmental problem. In particular, conventional systems are capable of greatly reducing the amount of vapor released to the environment. For example, such systems may be produced from which less than 0.5 grams of vapor will escape during the U.S. EPA and California Air Resource Board 3-day test. However, emerging environmental regulations require further reductions in the amount of fuel vapor emissions.
To date, only one very complex fuel system has been able to reduce fuel vapor emissions to meet emerging zero-evaporative emissions requirements. This system, described in SAE Paper 1999-01-0771, submitted by Honda RandD Company, Ltd. and incorporated herein by reference in its entirety, proposes that a system maintained at a vacuum emits no hydrocarbon vapor. However, the system described in the SAE paper only exposes the canister to the fuel tank during refueling. As a result, the system does not provide for capture of fuel vapor at any time other than during refueling. Moreover, the existing system requires extra valves between the tank and canister that must be actuated during refueling in order to effectuate operation of the system. Such extra valves provide additional failure modes and add to the expense of the system. Thus, a zero evaporative emissions fuel system relying on a vacuum is desired that is less complex than existing systems and also provides for vapor capture, storage and removal during other than refueling operations.
A zero emissions fuel system is disclosed. The system includes a fuel tank fluidly interconnected with a fuel vapor containment and absorption canister. The canister is further interconnected with an engine intake manifold via a purge solenoid valve that controls the extent to which a vacuum in the intake manifold affects the canister. The canister is also interconnected with the ambient environment via a vacuum relief valve and a refueling vent solenoid valve.
During normal operation of the engine, the purge solenoid valve valve, controlled by an electronic control module, opens a predetermined amount in response to engine demand, thereby allowing a vacuum in the intake manifold to draw vapor from the canister for consumption by the engine. At a predetermined vacuum pressure in the canister, the vacuum relief valve is drawn open, thereby allowing outside air to flow into the system through the vacuum relief valve to facilitate purging of the canister. If the canister pressure rises above the predetermined vacuum pressure, the vacuum relief valve closes. Since the fuel tank is in fluid communication with the canister, the canister and the fuel tank are always at the same pressure.
In one embodiment, the purge solenoid valve closes when the fuel mix in the intake manifold is too rich or if the intake manifold vacuum is insufficient to maintain vacuum in the tank and canister. A pressure relief valve connected to the canister operates to vent the fuel tank and canister if positive pressure rises above a predetermined setpoint due to extreme operating conditions.
In another embodiment, the vacuum relief and the pressure relief valves may be eliminated by electronically controlling the refueling vent solenoid valve between open and closed positions in response to predetermined pressure conditions. Thus, for example, the refueling vent solenoid would open all of the way for refueling, but may be opened only a little while the vehicle is running in response to a predetermined vacuum pressure or positive pressure. A control loop may control the extent to which the vent solenoid opens and closes.
Since the canister and the fuel tank are always at the same pressure, only one pressure sensing device need be utilized. Further, the canister is continuously connected with the tank such that it may absorb fuel vapor at any time, and is not limited to only absorbing vapor during fuel filling operation. The complexity of the system is reduced by eliminating several valves and conduits, thereby further reducing the cost of the system as well.